Artificial leathers have come to be widely used in the application fields of clothes, general materials, sports, etc. because their light weights and easiness of handling have been accepted by consumers. In such application fields, artificial leathers are required to meet both the sensory properties such as appearance and hand and the physical properties such as dimension stability. Artificial leathers excellent in appearance, hand, etc. are generally produced by a method including the conversion of microfine fiber-forming fibers into microfine fibers by removing one component therein. A known method generally used for producing artificial leathers which includes the conversion of fibers into microfine fibers are roughly composed of a step (1) in which microfine fiber-forming fibers made of two kinds of polymers having different dissolving properties are made into staple fibers, a step (2) in which the staples fibers are formed into a web by using a carding machine, crosslapper, random webber, etc., a step (3) in which the web is made into an entangled nonwoven fabric by entangling the fibers by a needle punching, etc., a step (4) in which a solution or emulsion of an elastic polymer such as polyurethane is impregnated into the entangled nonwoven fabric and coagulated, and a step (5) in which the microfine fiber-forming fibers are converted into microfine fibers by removing one of their components. The step (4) and step (5) may be carried out in reverse order. By such production methods, soft artificial leathers composed of microfine fibers are obtained.
Unlike a method of using short fibers, a method of using long fibers does not need a series of large apparatuses such as a raw fiber feeder, an apparatus for opening fibers, a carding machine and a cross lay machine. In addition, a nonwoven fabric made of long fibers is advantageous to a nonwoven fabric made of short fibers because of its high strength.
Nonwoven fabrics made from two kinds of long microfine fibers have been mainly produced by a method in which microfine fiber-forming long fibers composed of two or more kinds of incompatible polymers are made into a nonwoven fabric and then the microfine fiber-forming long fibers are converted into microfine fibers by splitting and dividing along the lengthwise direction through the interface between the polymers. However, it is very difficult to split or divide uniformly. Therefore, the obtained nonwoven fabric of long microfine fibers is mainly used in the production of grain-finished artificial leathers, but not suitable for the production of suede-finished artificial leathers. On the other hand, nonwoven fabrics made from a single kind of long microfine fibers have been produced by a method in which microfine fiber-forming long fibers composed of two or more kinds of incompatible polymers (microfine fiber-forming polymer and removable polymer) are made into a nonwoven fabric and then the removable polymer is removed from the multi-component fibers. For example, sodium hydroxide is used for removing polyester, formic acid is used for removing polyamide, and trichloroethylene or toluene are used for removing polystyrene and polyethylene.
The degree of water solubility of polyvinyl alcohol (PVA) which is known as a water-soluble polymer can be adjusted by changing its backbone, molecular structure and molecular form or by modifying the polymer. Further, PVA may be made thermoplastic, i.e., melt-spinnable. It is also known that PVA is biodegradable. Recently, it is imperative for the protection of the global environment to how harmonize synthetic chemicals with the nature. In this connection, PVA and PVA-based resin having such properties attract great attention.
Various artificial leathers having a natural leather-like softness have been proposed. For example, a proposed leather-like sheet is produced by impregnating a polyurethane resin into a nonwoven fabric made of microfine fibers of 1 D or less, wet-coagulating the polyurethane resin to obtain a substrate, and laminating the substrate with a film prepared by coating a polyurethane resin on a release paper. In another proposal, a leather-like sheet is produced by applying a polyurethane solution on the substrate obtained in the same manner as above, wet-coagulating the polyurethane, and then gravure roll-coating a polyurethane/colorant composition. In still another proposal, a leather-like sheet is produced by impregnating a polyurethane resin into an entangled nonwoven fabric made of sea-island fibers, wet-coagulating the polyurethane resin, removing the sea component from the sea-island fibers by dissolution in a solvent, to obtain a substrate made of bundles of microfine fibers of 0.2 D or less, and then, subjecting the substrate to the surface finishing treatment mentioned above (for example, Patent Document 1). Although the proposed leather-like sheets have a softness resembling natural leathers, a grain-finished artificial leather which combines softness without resistance, hand with stiffness and fine bent wrinkles has not yet been obtained.
Further proposed is an artificial leather which is produced by impregnating a high-density nonwoven fabric with a binder resin in an amount smaller than usual (Patent Document 2). However, the proposed artificial leather has a surface with a poor softness and a low interlaminar peeling strength. Therefore, it is insufficient as the material for sport shoes which are used under severe conditions.
Still further proposed is a grain-finished artificial leather made of a nonwoven fabric of long fibers (Patent Document 3). In Patent Document 3, it is described that the strain markedly caused during the entangling treatment of long fibers can be relieved by intendedly cut the long fibers during the entangling treatment by needle punching, thereby exposing the cut ends of fibers to the surface of the nonwoven fabric in a density of 5 to 100/mm2. It is also described that 5 to 70 fiber bundles are present per 1 cm width on the cross section parallel to the thickness direction of the nonwoven fabric of long fibers, i.e., the number of fiber bundles which are oriented by needle punching in the thickness direction is 5 to 70 per 1 cm width of the cross section. It is further described that the total area occupied by fiber bundles on a cross section perpendicular to the thickness direction of the nonwoven fabric of long fibers is 5 to 70% of the cross-sectional area. Although the long fibers are cut to an extent so as to achieve the intended properties, many long fibers are required to be cut to make the nonwoven fabric of long fibers into the proposed structure. Therefore, the advantages of using long fibers that the strength of nonwoven fabric is enhanced because of their continuity are significantly reduced, thereby failing to effectively use their advantages. To cut the fibers on the surface of nonwoven fabric evenly, the entangling treatment should be performed by repeating the needle punching many times under conditions severer than usual, thereby making it difficult to obtain the high-quality structure of long-fiber nonwoven fabric aimed in the present invention.
It is also known that a substrate for artificial leathers having a natural leather-like softness is obtained by impregnating a binder resin to an entangled nonwoven fabric of microfine fiber-forming fibers or bundles of microfine fibers and wet-coagulating the binder resin. However, a binder resin, particularly in the form of aqueous emulsion, present on the surface of the substrate for artificial leathers in a high concentration inhibits the adhesion of a surface layer (grain layer) to the surface of the substrate, thereby making the production of a grain-finished artificial leather having a high peeling strength difficult. For example, Patent Document 4 teaches that when blowing hot air for drying on to one of the surfaces of an entangled nonwoven fabric made of bundles of microfine fibers which is impregnated with an aqueous emulsion of a binder resin, the binder resin migrates mainly into the blown surface and the migration into the other surface is prevented. However, the aqueous emulsion of a binder resin cannot be completely prevented from being present on the non-blown surface by merely preventing the migration. Therefore, a substrate for artificial leathers made of microfine fibers which is free from a binder resin on the surface portion has not yet been obtained.    [Patent Document 1] JP 63-5518B (pages 2-4)    [Patent Document 2] JP 4-185777A (pages 2-3)    [Patent Document 3] JP 2000-273769 (pages 3-5)    [Patent Document 4] JP 54-59499A (pages 1-2)